Friday, November 16, 2012

Unveiling Obscured Supermassive Black Hole Growth

In a previous post, we discussed the class of objects known as Active Galactic Nuclei (AGN). AGN are actively growing black holes with masses as large as 100 billion times that of our Sun. These supermassive black holes were once thought to be rare, but are now known to live at the centers of nearly all galaxies. While the majority of supermassive black holes are inactive (at least in the present day), their extreme masses exert a strong gravitational pull on their surroundings capable of ripping a star apart if it wanders too close. While black holes occasionally grow by this violent process, they are more commonly fed by interstellar gas that first settles into a rapidly rotating disk around the black hole before being accreted (eaten). 

Artist's illustration of the accretion disk and jets surrounding
a black hole. Credit: Cosmovision, Dr. Wolfgang Steffen.
For an animated version, click here.
It is this accretion disk and other features in its vicinity that make actively accreting supermassive black holes visible and that make AGN among the most luminous objects in the Universe across much of the electromagnetic spectrum. The accretion disk itself gives off bright ultraviolet and optical/visible emission capable of outshining the light from the AGN’s host galaxy. A corona surrounding the accretion disk is thought to be responsible for the bright X-rays emitted by AGN, and in approximately 15% of cases, jets launched near the supermassive black hole give rise to bright radio emission. Incidentally, it was this combination of bright radio emission and bright, yet extremely compact, optical emission that gave AGN their other well-known name: quasars, or “quasi-stellar radio sources”.

Obscured AGN
The combination of bright ultraviolet, optical, X-ray, and occasionally radio emission should give astronomers plenty of ways to identify AGN. However, actively accreting supermassive black holes appear to be surrounded not only by an accretion disk of hot gas, but also by a donut-shaped torus of colder gas and dust (astronomer lingo for very small solid particles in space). This torus lies further from the black hole than the accretion disk, and while its origin and properties are still an active field of research, one thing is clear. If we are lucky enough to view an AGN from above or below the torus (e.g., though the hole in the donut), we get a clear view of the accretion disk and its surroundings. If, however, the torus is positioned so that we must look through it to see the accreting black hole, the picture changes. 

Artist's illustration of the dusty torus that surrounds an
AGN's accretion disk.  Credit: NASA/CXC/M.Weiss
Dust, it turns out, is very good at blocking UV and optical light, and gas is very good at blocking X-ray light. When we view an AGN through the torus, much of the light we would normally see is therefore missing, or significantly weakened. We call such unfortunately aligned sources ‘obscured AGN’. If these sources were rare, this would not be a big problem for our study of supermassive black hole growth. However, in the local Universe, obscured AGN are four times more common than unobscured AGN, and some studies point to an even higher fraction of obscured AGN at earlier times. In order to study how and why supermassive black holes grow, we must first find ways to identify these obscured AGN.

Finding Obscured AGN
Thankfully, not all obscured AGN remain entirely hidden. Radio emission is relatively insensitive to dust and gas, leading to fairly complete samples for the 15% of AGN with radio jets. Furthermore, while X-rays can be blocked by gas, it takes a lot to fully block the X-ray emission observed by the current generation of X-ray satellites. Deep X-ray observations by Chandra and XMM-Newton can therefore detect many sources that would be missed in the UV and optical, and recent X-ray missions like Swift, Integral, Suzaku and NuSTAR are opening our window on the heavily obscured Universe by probing more energetic X-rays that are even harder to obscure. Certain lines in the optical spectra of AGN are also emitted in a region beyond the torus, and can be used to identify obscured AGN missed by other techniques. 

And then there is the torus itself. When the dust in the torus absorbs the UV and optical light, it heats up to temperatures as high as 1500K (~2200 degrees Fahrenheit). While this may seem quite hot, it is several times cooler than the surface of the Sun. As such, the torus does not emit in the optical like a star might, but at lower energies in the infrared. (Note: at approximately 100 degrees Fahrenheit, we also emit our own infrared or ‘thermal’ emission). While warm dusty objects in the Universe are therefore faint in the UV and optical, they can be bright in the infrared, and the same is true for AGN. The torus that obscures our view of the AGN in the UV, optical, and X-ray in fact provides us with a way of identifying luminous AGN using the infrared satellites Spitzer and WISE

Visible (left) and infrared (right) images of a person with their hand in a bag.  While the visible light is blocked by the bag, the man's infrared emission passes through the bag.  Similarly, the ultraviolet and optical light absorbed by an AGN's torus warms the dust in the torus itself, which then produces infrared radiation. Credit: NASA/IPAC

Why Obscured AGN Matter
Obscured AGN make up the biggest fraction of the total AGN population, so it is crucial that we be able to detect and study these elusive sources if we hope to understand when, where, and why supermassive black holes formed. However, obscured AGN provide astronomers with another advantage. Unlike bright unobscured AGN, whose UV and optical light outshines the light from their host galaxies, the host galaxies of obscured AGN can often be seen quite clearly with minimal interference from the AGN itself. This is crucial, as one of the major open questions in astronomy concerns the relationship between AGN and their hosts. 

The Antennae Galaxies, the nearest example of a major
galaxy merger.  Credit: NASA/ESA and the Hubble Heritage Team 
As recently as 12 years ago, astronomers believed that AGN and their host galaxies grew and evolved independently of one another. However, the surprising discovery of a tight correlation between the mass of a supermassive black hole and the mass of its host galaxy’s bulge (the so-called 'M-sigma relation') indicates that the evolution of galaxies and their supermassive black holes are tightly coupled. One possible explanation for this connection is the merger of two or more near-equal-mass galaxies, a common outcome, particularly in the early Universe. These major galaxy mergers are thought to drive gas and dust into the central regions of a galaxy, fueling both star-formation and black hole growth. If emission from the supermassive black hole then shuts off both of these processes when the AGN reaches a certain luminosity (a process astronomers call ‘feedback’), the bulge and supermassive black hole cease to grow and end up with masses that are related to one another. However, there is increasing evidence that this scenario may only be important at high luminosities and/or at high redshifts (e.g., in the early Universe). This is where CANDELS come in. 

Not only were AGN most active when the Universe was only ~3 billion years old, but the major merger scenario proposed to explain the correlation between bulge and black hole mass may also be most relevant at this time. If light traveled infinitely fast, we would have no way of knowing what AGN or their hosts looked like ~11 billion years ago. Thankfully, however, light has a finite speed, so the more distant an object is, the longer its light has been traveling to us, and the younger it was when that light was emitted. Along the way, light also gets stretched by the expanding Universe, so that light that left a distant galaxy in the optical is shifted into the near-infrared by the time it reaches us. In practice, this means that the CANDELS deep near-infrared data gives us a snapshot of the optical light that was emitted by distant AGN and their host galaxies when the Universe was still quite young. For bright unobscured AGN, CANDELS therefore allows us to study accreting supermassive black holes in the early Universe. For fainter AGN and luminous obscured AGN, however, CANDELS is providing us with the first optical images of AGN hosts in the early Universe. What will we see?  Are AGN hosts undergoing major galaxy mergers, or are other processes driving the growth of supermassive black holes and their host galaxies? Our first study of the host galaxy properties of X-ray selected AGN in CANDELS is discussed here. Stay tuned for more results! 

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